Day_Lecture_10 - Chemistry 307 Chapter 10 Nuclear Magnetic...

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1 Chemistry 307 Chapter 10 – Nuclear Magnetic Resonance Nuclear magnetic resonance (NMR) spectroscopy is one of three spectroscopic techniques that are useful tools for determining the structures of organic compounds. [We will learn about infrared (IR) spectroscopy in chapter 11 and about ultraviolet/visible (UVVis) spectroscopy in chapter 14.] Spectroscopic techniques probe the energy differences between two “states” in a molecule by irradiating it with electromagnetic radiation of known frequency. We can observe “transitions”, i.e., signals, when the incident radiation has the exact frequency, (that is a Greek nu) so that the energy of the photon, h , matches the energy difference, E, between the two states, E = h (Figure 10.1). Spectroscopic techniques are non- destructive; the excited molecules decay back to the ground state without decomposition. Mass spectro metry (chapter 11) is not a spectro scopic technique; it leads to the destruction of the sample. QuickTime™ and a Photo - JPEG decompres or are ne ded to se this picture. Photons of different energies can probe different types of transitions (Figure 10.2). Different spectroscopic methods use different units to characterize the energies of the photons applied. The units are all related to the general equation, linking energy ( E) to frequency, (unit: s 1 or Hz, named after Hertz). E = h UVVis spectroscopy uses wavelength, (unit: nm), to characterize the energy of the photon . Wavelength is related to frequency by = c . E = h c IR spectroscopy uses wave number, 1/ (unit: cm 1 ) E = h c Nuclear magnetic transitions are probed with radio waves. Compared to other spectroscopic techniques NMR has an additional complication: the
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2 energy differences between nuclear states and the “resonance frequenc y” are not constant, but depend on the magnetic field, H 0 , at which the spectrometer operates i.e., E, H 0 . At a magnetic field, H 0 = 70 kGauss, 1 H nuclei resonate at 300 MHz. The energy difference, E, between 1 H nuclear levels at 70 kGauss is only ~3x10 –5 kcal mol –1 . E Therefore, it is not sufficient to give the frequency at which an NMR transition occurs; we have to specify both the photon frequency and the magnetic field strength to describe our results unambiguously. Because of the very small energy difference between the two nuclear spin levels transitions between them are very fast: both levels are in equilibrium (see Chapter 2). = –RTlnK or lnK = G°/RT The very low energy difference between our nuclear spin levels ( E ~3 10 –5 kcal mol –1 ) means that the population differences between the nuclear spin levels are very small, typically much less than 1%. We will focus our discussion on the magnetic resonance of
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Day_Lecture_10 - Chemistry 307 Chapter 10 Nuclear Magnetic...

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